So your argument here is that while the software can be open source, it matters less, if whatever the software does isn't actually an open standard? Wouldn't "being open source with own custom protocol" essentially be as open as "open source or not, but software implements open standards" anyways?
Many protocols (even open) are complex, and partially undocumented.
It would be nice to have both (open source and open protocol), but I kind of agree that if we should push for one, an open (decently explained) standard will probably be easier, simpler and with longer term impact, not to mention the interoperability benefits between countries.
Especially for the use case they’re talking about. It makes sense to have open standards for something like filing taxes so many companies can compete.
Having source code for the tax system itself is interesting, but I think the market for “run software for processing incoming taxes for polish citizens” is exactly one.
Unless they expect pull requests, which could be fun, but as OSS maintainers know, it’s a ton of work and boy would there be a ton of spam on something like this.